Preheat the oven to 350F. Line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl, whisk together the flour, cinnamon, baking soda, salt, and nutmeg.
In a large mixing bowl or the bowl of a stand mixer, beat the butter on medium speed until creamy. Add the sugars. Beat until well combined, about 2 minutes.
Add the egg and vanilla, and beat until fluffy, about 2 minutes.
Scrape down the bowl, and add the mashed banana. Beat until well combined. Add the flour mixture and beat on low speed just until incorporated. Mix in the oats and chocolate chips. Scoop the dough into 2-tablespoon-sized mounds onto the lined baking sheet, spacing the cookies about 2 inches apart. (For flatter cookies, gently press the mounds down with lightly greased fingers.)
Bake for 12 to 15 minutes or until golden around the edges and the centers appear dry. Let the cookies cool on the pan for a few minutes, then transfer to a wire rack to cool completely. Continue baking the remaining dough. Cool cookies can be stored in an airtight container for up to 4 days, or frozen for a few months.
The cookies will not spread or flatten a lot as they bake, so be sure to create the shape you want beforehand by flattening them if you want a flatter cookie.
Measure your flour correctly! Adding too much flour can really affect the taste and texture of the cookies. The easiest way to measure flour is by using a scale for the most accurate results. If you don’t have one, then fluff your flour with a spoon, spoon it into your measuring cup, and use a knife to level it off.
Make sure your brown sugar is soft and moist for the best cookies. If your brown sugar has hardened, don’t worry, it’s an easy fix! I wrote a detailed post on how to soften brown sugar.
Don’t over-bake the cookies, or they’ll become hard as they’ll continue to bake as they cool. The middle should still be soft when you pull them out.
Room temperature eggs will mix more evenly into the creamed butter. Place your egg into a bowl of warm water for a couple of minutes if you’ve forgotten to take it out of the fridge beforehand. Doing so will quickly bring it to room temperature.
Make sure to line your baking sheet with parchment paper or a silicone mat so your cookies don’t stick. It also prevents the bottom of the cookies from getting too brown and crispy.
Mix-ins such as dried fruit or chopped nuts make a great addition to cookies.